It is known that the rupture in metals at high temperatures occurs at the grain boundaries. It is also known that if a turbine blade is formed of a metal which has a single-crystal structure with no grain boundary and which is subjected to appropriate heat treatment, the creep rupture strength of this blade at high temperatures is remarkably improved. On the basis of this recognition, United Technologies Corporation proposed Alloy 444 (disclosed in the U.S. Pat. No. 4,116,723), Alloy 454 (disclosed in the U.S. Pat. No. 4,209,348) and Alloy 203E (disclosed in the U.S. Pat. No. 4,222,794), Air Research Corporation proposed NASAIR 100, and Canon Muskegon Corporation proposed CMSX-2 (disclosed in the Japanese Pat. Unexamined Publication No. 57-89451) and CMSX-3 (disclosed in the Japanese Pat. Unexamined Publication No. 59-190342). All of these alloys are heat resistant nickel-base super alloys only for single crystals.
In addition to them, heat resistant single-crystal nickel-base super alloys are also proposed in the British Pat. No. 1,557,900, British Pat. No. 2,159,174A, European Pat. No. 0063511Al, U.S. Pat. No. 4,402,772, etc.
The above-mentioned single-crystal alloys have by far superior creep rupture strength to ordinary polycrystal alloys. In practical applications, however, the development of alloys that provide higher creep rupture strength and excellent oxidation resistance is desired for the purpose of improving the efficiency of gas turbine engines; providing that the use of very expensive alloying elements, such as rhenium, is not desirable for the development of these alloys.
Conventionally, the improvement of the creep rupture strength of a single-crystal alloy depends mainly on an increase in the amounts of added wolfram tungsten, and tantalum as alloying elements. Unfavorable phenomena such as precipitation of detrimental phases occur if the added amounts are too large. These unfavorable phenomena make it difficult to develop alloys with high creep rupture strength. For example, Alloy 444, Alloy 454, etc. which were developed formerly do not provide sufficiently high creep rupture strength. Alloy 203E and the alloy disclosed in the British Pat. No. 1,557,900 contain rhenium which is an expensive alloying element. NASAIR 100 was developed to improve the creep rupture strength. It was found that in the case of this alloy, detrimental phases, such as .alpha.-wolfram phase and .mu.-phase, precipitate due to high wolfram contents, resulting in a decrease in the creep rupture strength. Similarly, it seems that the .alpha.-wolfram phase, etc., precipitate due to high wolfram and tantalum contents in the alloy described in the British Pat. No. 2,159,174A. To prevent the precipitation of detrimental phases, such as .alpha.-wolfram phase, it is necessary to reduce the amounts of added wolfram, molybdenum, tantalum, etc. If, however, these added amounts are excessively reduced, the creep rupture strength decreases. CMSX-2 and CMSX-3 are alloys developed to prevent the precipitation of the .alpha.-wolfram phase, .mu.-phase, etc. and to obtain a stable microstructure. However, the creep rupture strength of these alloys is not sufficiently high. The creep rupture strength of the alloys disclosed in the European Pat. No. 0063511A1 and U.S. Pat. No. 4,402,772 is not sufficiently high, either.
Turbine blades are parts subjected to high temperatures and oxidation resistance is one of the important properties that turbine blades are required to provide. In general, oxidation resistance is improved by increasing the amounts of alloying elements, such as chromium and aluminum. To stabilize the alloy microstructure and obtain high creep rupture strength, however, the amounts of chromium and aluminum are limited to narrow ranges. For this reason, it is not easy to obtain good oxidation resistance.
To develop an alloy that possesses microstructural stability and excellent creep rupture strength without using expensive alloying elements such as rhenium, the inventors of the present invention examined the amount of each alloying element added and the composition balance of the alloying elements. As a result, following alloy was discovered: a single-crystal nickel-base heat-resisting superalloy being composed of 4-10% chromium, 4-6.5% aluminum, 4-10% wolfram, 4-9% tantalum, 1.5-6% molybdenum by weight and the balance of nickel and incidental elements and satisfying the conditional expression W/2+Ta/2+Mo=9.5-13.5%, as disclosed in the Japanese Pat. Unexamined Publication No. 62-116748. The inventors also discovered an alloy obtained by adding not more than 12% cobalt to this alloy (disclosed in the Japanese Pat. Unexamined Publication No. 62-290839) as an alloy in which the creep rupture strength of this alloy is further improved. These alloys that are excellent in the creep rupture ductility and stability of the micro structure will be able to extend the life of gas turbine engine blades if their oxidation resistance is further improved.